The present invention relates generally to a GPS receiver, and more particularly to a GPS receiver operating in conjunction with a base station.
The Global Positioning System (GPS) is a satellite-based system developed by the U.S. Department of Defense to give accurate positional information to a GPS receiver anywhere in the world. A properly equipped GPS receiver may therefore be used in any setting in which a positional fix is desired, and typically yields positional coordinates in three dimensions. The GPS system is enabled by a satellite orbital constellation made up of 24 or more satellites orbiting the earth in 12-hour orbits. The satellites are arranged in six orbital planes, each containing four satellites. The orbital planes are spaced sixty degrees apart, and are inclined approximately fifty-five degrees from the equatorial plane. This constellation ensures that from four to twelve satellites will be visible at any time at any point on earth.
A GPS receiver is capable of determining a positional fix in three dimensions. This may be achieved if signals are being received from four or more GPS satellites. The received satellite signals each contain an identifier unique to that particular satellite. These identifier codes are commonly called Gold codes and allow a GPS receiver to discriminate between signals from different satellites. Also contained within the signals are satellite ephemeris data containing information such as an orbital configuration and a satellite time (all GPS satellite signals contain a common, synchronized time). This time signal allows a GPS receiver to detect a time of receipt and therefore measure a transit time of the signal. In turn, the transit time enables a GPS receiver to determine a distance (termed a pseudorange) to the satellite. The pseudorange D to a single satellite is shown in FIG. 1. The pseudorange does not give a simple distance to a particular spot. The pseudorange from a particular satellite to the GPS receiver may describe a circle on the Earth (if the satellite is directly overhead), or, as is more likely, may describe an arc such as a parabola upon the curved surface of the earth.
FIG. 2 shows three GPS satellites A, B, and C. Each satellite gives a pseudorange curve, with the three intersecting at one point. These three (or more) curves may be solved to find a positional fix of the intersecting point (i.e., a positional fix of the GPS receiver). Three such GPS pseudoranges allow a location on the Earth""s surface to be determined, while four yield a three dimensional determination.
Another important satellite characteristic is a Doppler signature. Electromagnetic waves appear to be increased or decreased in frequency when the wave source is moving relative to a receiver. This is termed the Doppler effect. The Doppler effect is observed by GPS receivers, and is the most pronounced when a GPS satellite is at a low angle above the horizon. A GPS satellite directly overhead is nearly motionless (i.e., zero Doppler) with respect to an observer (satellite A in FIG. 2), while a satellite B low on the horizon, traveling at the same velocity as satellite A, will be moving faster with respect to the observer. Each visible satellite therefore may be moving at a different speed relative to a point on the ground, and as a result may have a unique Doppler signature. Therefore, a Doppler signature of satellite C will be much larger than one for satellite B. These Doppler signatures can be used to compute user velocity vector.
The GPS satellites transmit data to be used by GPS receivers, including identification data, satellite position data (ephemeris data), and satellite clock correction data. The GPS signal includes a carrier signal that is bi-phase modulated with a 1023 bit long Gold spreading code at a 1.023 MHz chip rate (0.001 second repeat interval). It is also modulated by data bits at a 50 bits per second (BPS) rate (transmitted at a rate of twenty milliseconds per data bit). The 50 BPS data includes information for determining a GPS-based time (i.e., a clock time of the GPS satellite) and information for determining geographical location.
Detailed information on the data contained within the GPS signal is available in Interface Control Document ICD-GPS-200, revised in 1991, published by Rockwell International Corporation and incorporated herein by reference.
A prior art GPS receiver 100 is shown in FIG. 3 and is described in U.S. Pat. No. 5,663,734 to Krasner. The prior art GPS receiver 100 includes an antenna 102, a down converter 105, a reference oscillator 107, a clock generator 112, an analog-to-digital converter (A/D) 114, a dynamic random access (DRAM) memory 118, a memory sequencer 122, a mixer 127, a numerically controlled oscillator (NCO) 133, a dual port stack RAM 136, a digital signal processor (DSP) 140, and a local DSP results RAM 144.
The prior art GPS receiver 100 receives a GPS signal at the antenna 102 and down-converts it into an intermediate frequency signal (IF signal). The IF signal is fed to the A/D 114, where it is converted into a digital GPS signal. The resulting digital GPS signal is then stored continuously into the DRAM 118. The DRAM 118 of the prior art is very large, typically involving about 16 Mbits (16 million bits) of memory. Such a large memory size is typically needed to capture about 1 second of GPS data. Once an entire 1 second sample is stored in the DRAM 118, it is typically read out and processed by a programmable signal processor for the purpose of extracting the signal pseudoranges and Doppler shift characteristics for all visible GPS satellite signals stored in the DRAM 118.
The 1 second digital GPS data segment is post-processed, in a sequential fashion. The prior art scheme does not process the incoming signal in real time. One reason for this is that by storing a large sample of GPS data (i.e., 1 second), the DSP 140 does not need a throughput capable of keeping up with the incoming data. The prior art GPS receiver 100 typically operates by collecting a 1-second sample and then taking about 3 to 10 seconds to process the sample. During the processing time, the incoming GPS signal is simply not stored or used. The result is certain coarseness to the GPS positional fix. The positional fix may have a time lag and may not accommodate frequent positional changes, which is especially important in mobile applications. These drawbacks may be acute in applications where the positional fixes are used to gather data, such as in geographical survey or agricultural sampling applications.
Another drawback of the prior art is that the large size of the DRAM 118 incurs unneeded cost in a GPS receiver or a device incorporating a GPS receiver. Such a device might be, for example, a cellular phone or a pager.
In the prior art processing arrangement, in order to process data from 4 to 8 GPS satellites (generally 4 to 8 GPS satellites are visible at any time at any point on the Earth), a prior art GPS receiver operating in a sequential fashion would need to have a processor throughput on the order of about 500 MIPs (Millions of Instructions Per Second). This is, of course, a very high throughput requirement, and such a processor, if available, would be expensive and would generate a lot of heat.
There remains a need in the art, therefore, for a GPS receiver having a reduced memory requirement that can process incoming GPS data in real time.